Method of manufacturing carbon-coated electrode active material and electrode active material manufactured by the method

ABSTRACT

Disclosed is a method of carbon-coating a secondary battery active material and a second battery active material produced by the method. The method of producing a carbon-coated battery active material involves mixing a carbon precursor with liquid carbon dioxide to produce a coating solution comprising a carbon material, coating a battery active material with the carbon material by applying the coating solution to the battery active material, and sintering the coated battery active material to obtain the carbon-coated battery active material.

BACKGROUND

1. Field

The following description relates to a method of uniformly coating asurface of a secondary battery active material with a carbon layer, anda secondary battery active material manufactured thereby.

2. Discussion of Related Art

Recently, in order to reduce dependence on petroleum and fundamentallyreduce greenhouse gases such as carbon dioxide, eco-friendly plug-inhybrid electric vehicles (PHEVs), electric vehicles (EV), and energystorage systems (ESS) using lithium secondary batteries as energysources have been competitively developed. In addition, it is expectedthat demands for medium-sized and large-sized secondary batteries willsignificantly increase in various fields, such as robots, backup power,medical equipment, and the like. Accordingly, research and developmentrelated thereto are being actively executed. As the demand for highcapacity batteries increases, evaluation of a high energy density, ahigh capacity, and safety becomes important factors to consider inmanufacturing batteries.

Currently, lithium composite metal oxides, such as lithium cobalt oxide(LiCoO₂), lithium manganese oxide (LiMn₂O₄), and lithium nickel oxide(LiNiO₂) are widely used as a material for a positive electrode of asecondary battery. Although LiCoO₂ has excellent high voltage andcharge/discharge characteristics and is most widely used as a positiveelectrode active material, LiCoO₂ is not a suitable positive electrodematerial of a large-sized secondary battery for PHEVs or power storagesdue to high manufacturing cost since cobalt (Co), a raw materialthereof, is rare, geographically concentrated, and expensive, and due tosafety problems since cobalt (Co) is toxic and causes environmentalpollution when discharged to outside and is unstable with respect totemperature and causes explosion at high temperature. Accordingly, inorder to resolve such issues, studies on fabrication of a positiveelectrode active material represented by a formula of LiMn₂O₄ or LiNiO₂containing manganese (Mn) or nickel (Ni), which have relatively abundantreserves, have been conducted. However, in the case of LiMn₂O₄, astability problem since Mn is dissolved in an electrolyte, and a shortlifespan problem since deterioration thereof progresses at hightemperature exists, and in the case of LiNiO₂, problems of batterycapacity seriously being degraded during a charging and dischargingprocess since a crystal structure thereof collapses and of low thermalstability exist. Accordingly, in order to achieve large-sized and massproduction of lithium secondary batteries, demand for a novel positiveelectrode active material that achieves high performance and provideshigh safety and high reliability is being increased.

Meanwhile, a variety of carbon-based negative electrode activematerials, such as artificial graphite, natural graphite, hard carbon,or soft carbon, are widely used as materials of negative electrodes oflithium secondary batteries. The use of a carbon-based negativeelectrode active material is characterized by various positivequalities, such as a similar operation voltage to metallic lithium,structural stability, excellent reversible cycling performance for along time. However, the practical application of a carbon-based negativeelectrode active material poses a challenge in that a battery formedthereof has a low energy density per unit volume due to a low density ofa carbon-based material. In addition, since an oxidation/reductionpotential of the carbon-based negative electrode active material islower than a potential of Li/Li⁺ by about 0.1 V, decomposition may occurdue to a reaction with an organic electrolyte used for fabricating abattery, a solid electrolyte interface (SEI) film can form on a surfaceof carbon-based material due to a reaction with lithium, and therebycharging/discharging characteristics may be degraded. In particular, inan application requiring high rate capability, such as the EV, sinceresistance increases during insertion/elimination of lithium due to theformation of the SEI film, a problem of the high rate capabilitydeteriorating may exist. Further, the carbon-based negative electrodeactive material imposes a safety concern in that lithium havingextremely strong reactivity is precipitated on a surface of a negativeelectrode during high rate charging/discharging operations and reactswith an electrolyte and a positive electrode material, potentiallyresulting in an explosion.

Accordingly, in order to achieve mass production of large-sized lithiumsecondary batteries, a demand exists to develop a novel cathode activematerial and anode active material that achieves high performance andprovides safety and reliability. Recently, various types of cathodeactive materials including phosphate-based material having a formula ofolivine-structured LiMPO₄ (M is Mn, Fe, Co, or Ni) which provides highperformance, safety, and reliability, and silicate-based Li₂NSiO₄ (N isFe or Mn), and anode active material including a lithium titanium oxide(LTO) having a formula of spinel-structured Li₄Ti₅O₁₂, and anatase-typeTiO₂, are attracting much attention as potential active materials ofsuch medium and large-sized secondary batteries.

The cathode electrode active material formed of olivine-structuredLiMPO₄ (M is Mn, Fe, Co, or Ni) or silicate-based Li₂NSiO₄ (N is Fe orMn) has many advantages, compared to existing LiCoO₂. When M and N areFe (LiFePO₄ and Li₂FeSiO₄), there are positive qualities, such asextremely low material price compared to LiCoO₂ due to abundant reservesand a low cost of Fe, low-toxicity and eco-friendly characteristics, andextremely low risk of explosion at high temperature due to structuralstability. In addition, since theoretical output densities thereof arerelatively high, that is, 170 mAh/g (LiFePO₄) and 330 mAh/g (Li₂FeSiO₄),battery capacity per unit mass may be increased. However, compared tothe existing LiCoO₂, the LiFePO₄ and Li₂FeSiO₄ cathode active materialmay not satisfy electrochemical properties required for the cathodeactive material of lithium secondary batteries. This is because onlylithium ions disposed on surfaces of particles are used and lithium ionsdisposed in center portions of particles are not used sinceinsertion/elimination of lithium ions during charging and discharging isextremely slow and a diffusion rate of lithium ions in the cathodeactive material is very low. In addition, battery performance maysignificantly deteriorate due to an overvoltage phenomenon occurringduring high rate charging/discharging operations since electricalconductivity of the cathode active material is very low.

Meanwhile, in the cases of LTO and TiO₂, since oxidation/reductionpotentials thereof are in the range of 1.5 to 1.7 V, that is, higherthan a potential of Li/Li⁺, there is little possibility of decompositionof an electrolytic solution and consequential formation of an SEI film,which occurs as a problem in the carbon-based anode active material.

In addition, due to the high oxidation/reduction potentials, there islittle possibility of precipitation of metallic form lithium. Theprecipitation of metallic form lithium generates a problem during highrate charging/discharging operations of the carbon-based anode activematerial. Accordingly, LTO and TiO₂ may be extremely safe during thehigh rate charging/discharging operations and may be utilized as powersources of PHEVs, EVs, ESSs, power tools, or uninterruptible powersupplies. In addition, theoretical densities are about 3.48 g/cm³ (LTO)and 4.23 g/cm³ (TiO₂), which are much higher than that of thecarbon-based negative electrode active material. Accordingly, LTO andTiO₂ are spotlighted as a novel anode active material of large-sizedsecondary batteries, such as in the EVs or the ESSs, due to high safety,excellent high-rate charging/discharging characteristics, and highreliability thereof.

However, since LTO and TiO₂ have very low electrical conductivity (10⁻¹³S cm⁻¹) and lithium-ion conductivity, an insertion/elimination rate oflithium ions during charging and discharging is extremely low whenmicron-sized LTO and TiO₂ particles (10 to 100 μm; aBrunauer-Emmett-Teller (BET) specific surface area of 2-5 m²/g) areused. Accordingly, charge/discharge capacities of LTO and TiO₂ are onlyabout 70% of their theoretical capacities, and the LTO and TiO₂ have notbeen widely used as the anode active material of lithium secondarybatteries.

Currently known technologies for increasing electrical conductivity byadding a conductive material to LiMPO₄ (M is Mn, Fe, Co or Ni), Li₂NSiO₄(N is Fe or Mn), LTO, or TiO₂, may be classified into a technology ofadding carbon having excellent conductivity by a solid-phase methodbefore formation of particles, a technology of coating a surface of asynthesized active material with a carbon precursor (sugar, cellulose,citric acid, or the like) by a liquid-phase method, such as ahydrothermal method, an emulsion method, or a sol-gel method, atechnology of coating active material particles with carbon using avapor-phase method, such as a chemical vapor deposition (CVD) method ora spray pyrolysis. However, when the active material is coated withcarbon obtained by the above-described methods, the uniformity of thecarbon coating the active material is very low. Accordingly, it isinevitable to add an excess of carbon to increase electricalconductivity of the active material. Since a density of carbon issignificantly lower than a density of the active material, there is aproblem in that an energy density per unit volume of the battery may besignificantly lowered when an electrode is formed of the active materialcoated with carbon by the above-described methods. In addition, wastesolution and waste organic solvent may be excessively generated as abyproduct, resulting in environmental pollution when the liquid-phasemethod is used. Accordingly, it is necessary to develop a carbon coatingmethod that enables a secondary battery active material to haveexcellent electrochemical properties by coating a lithium secondarybattery active material with a uniform carbon layer having a nano-scalethickness thereby improving electrical conductivity of the entirety ofthe active material particles and have excellent energy density per unitvolume by minimizing carbon particles which do not coat the activematerial particles, and enables a simplification of a waste water andwaste chemical material treatment process by using an eco-friendlymethod.

SUMMARY

This Summary is provided to introduce a selection of concepts in asimplified form that are further described below in the DetailedDescription. This Summary is not intended to identify key features oressential features of the claimed subject matter, nor is it intended tobe used as an aid in determining the scope of the claimed subjectmatter.

In one general aspect, a method of producing a carbon-coated secondarybattery active material involves forming a coating solution comprising acarbon precursor dissolved in liquid carbon dioxide, coating a surfaceof a secondary battery active material with the carbon precursor byimmersing the secondary battery active material in the coating solution,and sintering the secondary battery active material coated with thecarbon precursor.

The general aspect of the method may further involve removing the liquidcarbon dioxide after coating the secondary battery active material withthe carbon precursor and before sintering the secondary battery activematerial coated with the carbon precursor.

The carbon precursor may include at least one selected from the groupconsisting of sucrose octaacetate, fluorinated hydrocarbons,polyethylene glycol, acrylic acid, methacrylic acid, acrylamide, vinylpyrrolidone, glycidyl methacrylate, and styrene.

The carbon precursor may include at least one functional group selectedfrom the group consisting of a carboxyl group, a hydroxyl group, anepoxy group, an ester group, a thiol group, and a sulfonic group.

A concentration of the carbon precursor in the coating solution may bein the range of 5 to 40 wt %.

The coating solution may be produced at a temperature of 0 to 30° C.under a pressure of 30 to 200 bar.

The coating of the secondary battery active material may be performedunder a pressure of 30 to 200 bar.

The sintering of the secondary battery active material may be performedat a temperature of 200 to 800° C. for 1 to 3 hours.

The sintering of the secondary battery active material may be performedin an atmosphere comprising one or more gases selected from the groupconsisting of hydrogen, helium, neon, argon, krypton, xenon, and radon.

The sintering of the secondary battery active material may be performedin an atmosphere comprising a hydrocarbon gas.

The secondary battery active material may include at least one selectedfrom the group consisting of titanium dioxide, a compound represented byFormula 1, a compound represented by Formula 2, and a compoundrepresented by Formula 3, wherein:

LiMPO₄  [Formula 1]

(wherein M is one of manganese (Mn), iron (Fe), cobalt (Co), and nickel(Ni))

Li₂NSiO₄  [Formula 2]

(wherein N is iron (Fe) or manganese (Mn))

Li₄Ti₅O₁₂.  [Formula 3]

The secondary battery active material before sintering may include atleast one amorphous portion, and the amorphous portion may be changed toa crystalline portion during the sintering of the secondary batteryactive material.

In another general aspect, there is provided a secondary battery activematerial including a carbon coating layer coating a surface thereof,wherein a weight ratio of the carbon coating layer with respect to atotal weight is 1.6 to 5 wt %.

In another general aspect, a method of producing a carbon-coated batteryactive material involves mixing a carbon precursor with liquid carbondioxide to produce a coating solution comprising a carbon material,coating a battery active material with the carbon material by applyingthe coating solution to the battery active material, and sintering thecoated battery active material to obtain the carbon-coated batteryactive material.

The carbon precursor may include at least one selected from the groupconsisting of sucrose octaacetate, fluorinated hydrocarbons,polyethylene glycol, acrylic acid, methacrylic acid, acrylamide, vinylpyrrolidone, glycidyl methacrylate, and styrene.

The secondary battery active material may include particles with anaverage diameter between 10 to 100 μm, the particles including at leastone selected from the group consisting of titanium dioxide or a lithiumcomposite metal oxide including manganese, iron, cobalt, nickel ortitanium.

Other features and aspects will be apparent from the following detaileddescription, the drawings, and the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a flowchart illustrating an example of a method ofcarbon-coating a secondary battery active material using liquid carbondioxide.

FIG. 2 is an XRD pattern graph of LiFePO₄ particles beforecarbon-coating according to Embodiment 1 of the present description.

FIG. 3 is an SEM photograph of LiFePO₄ particles before carbon-coatingaccording to Embodiment 1 of the present description.

FIG. 4 is an XRD pattern graph of LiFePO₄ particles coated with carbonprecursors according to Embodiment 1 of the present description.

FIG. 5 is an SEM photograph of LiFePO₄ particles coated with carbonprecursors according to Embodiment 1 of the present description.

FIG. 6 is an XRD pattern graph of LiFePO₄ particles coated with carbonlayers after sintering according to Embodiment 1 of the presentdescription.

FIG. 7 is an SEM photograph of LiFePO₄ particles coated with carbonlayers after sintering according to Embodiment 1 of the presentdescription.

FIGS. 8A and 8B are HR-TEM photographs of LiFePO₄ particles coated withcarbon layers after sintering according to Embodiment 1 of the presentdescription.

FIG. 9 is an XRD pattern graph of LiFePO₄ particles beforecarbon-coating according to Comparative Example 1 of the presentdescription.

FIG. 10 is an SEM photograph of LiFePO₄ particles coated with carbonlayers after sintering according to Comparative Example 1 of the presentdescription.

FIGS. 11A and 11B are HR-TEM photographs of LiFePO₄ particles coatedwith the carbon layers after sintering according to Comparative Example1 of the present description.

FIG. 12 is an initial charge/discharge curve graph of LiFePO₄ particlesfabricated by a solid-phase method before carbon-coating.

FIG. 13 is an initial charge/discharge curve graph of LiFePO₄ particlescoated with carbon according to Embodiment 5 of the present description.

FIG. 14 is an initial charge/discharge curve graph of LiFePO₄ particlescoated with carbon according to Comparative Example 5.

FIG. 15 is a graph illustrating the charge/discharge capacity accordingto the number of cycles of LiFePO₄ particles coated with carbon, wherein(a) represents the charge/discharge capacity according to the number ofcycles of LiFePO₄ particles coated with carbon according to Embodiment5, and (b) represents the charge/discharge capacity according to thenumber of cycles of LiFePO₄ particles coated with carbon according toComparative Example 5.

FIG. 16 is a graph for describing changes in charge/discharge capacityaccording to the weight ratio of a carbon coating layer.

FIG. 17 illustrates a device for carbon-coating a secondary batteryactive material using liquid carbon dioxide according to an example ofthe present description.

Throughout the drawings and the detailed description, the same referencenumerals refer to the same elements. The drawings may not be to scale,and the relative size, proportions, and depiction of elements in thedrawings may be exaggerated for clarity, illustration, and convenience.

DETAILED DESCRIPTION

The following detailed description is provided to assist the reader ingaining a comprehensive understanding of the methods, apparatuses,and/or systems described herein. However, various changes,modifications, and equivalents of the methods, apparatuses, and/orsystems described herein will be apparent to one of ordinary skill inthe art. The sequences of operations described herein are merelyexamples, and are not limited to those set forth herein, but may bechanged as will be apparent to one of ordinary skill in the art, withthe exception of operations necessarily occurring in a certain order.Also, descriptions of functions and constructions that are well known toone of ordinary skill in the art may be omitted for increased clarityand conciseness.

The features described herein may be embodied in different forms, andare not to be construed as being limited to the examples describedherein. Rather, the examples described herein have been provided so thatthis disclosure will be thorough and complete, and will convey the fullscope of the disclosure to one of ordinary skill in the art.

Hereinafter, a method of carbon-coating a secondary battery activematerial using liquid carbon dioxide according to various examples and asecondary battery active material coated with carbon by the method willbe described in detail with reference to accompanying drawings.

The same reference numbers will be used throughout this specification torefer to the same or like components. Unless otherwise defined, allterms (including technical and scientific terms) used herein have thesame meaning as commonly understood by one of ordinary skill in the art.It will be further understood that terms, such as those defined incommonly used dictionaries, should be interpreted as having a meaningthat is consistent with their meaning in the context of the relevant artand will not be interpreted in an idealized or overly formal senseunless expressly so defined herein.

FIG. 1 illustrates an example of a method of carbon-coating a secondarybattery active material using liquid carbon dioxide.

Referring to FIG. 1, the method of carbon-coating the secondary batteryactive material using liquid carbon dioxide involves a process offabricating a coating solution (S10), a coating process (S20), a processof removing liquid carbon dioxide (S30), and a sintering process (S40).

In the process of fabricating a coating solution (S10), carbonprecursors are dissolved in liquid carbon dioxide to fabricate thecoating solution. In some embodiments, carbon precursors formed of anorganic material are introduced to a high-pressure solution vessel, thenliquid carbon dioxide is added thereto. With the formation of a mixtureof the liquid carbon dioxide and the carbon precursors, the carbonprecursors are dissolved in the liquid carbon dioxide to form thecoating solution.

As the carbon precursors, any organic material containing carbon may beused without limitation. In some embodiments, as the carbon precursors,one or more selected from the group consisting of sucrose octaacetate,fluorinated hydrocarbons, polyethylene glycol, acrylic acid, methacrylicacid, acrylamide, vinyl pyrrolidone, glycidyl methacrylate, and styrenemay be used.

In some embodiments, the carbon precursors may include a functionalgroup having a high degree of affinity with respect to a metal includedin active material particles. For example, the carbon precursors mayinclude at least one functional group selected from the group consistingof a carboxyl group, a hydroxyl group, an epoxy group, an ester group, athiol group, and a sulfonic group. When the carbon precursors include atleast one functional group having the high degree of affinity withrespect to a metal, the carbon precursors may be bonded to surfaces ofthe active material particles. Accordingly, even when the activematerial particles are exposed to the carbon precursors for a shortperiod of time, the surfaces of the active material particles may beuniformly coated with the carbon precursors.

A viscosity of liquid carbon dioxide may be about 0.1 centipoise (cp) atroom temperature, which is significantly lower than a viscosity of anormal organic solvent or water, which is about 0.5 to 1.5 cp. A surfacetension of liquid carbon dioxide may be 5 dyne/cm or lower at roomtemperature, which is significantly lower than a surface tension of thenormal organic solvent, which is about 25 to 30 dyne/cm, or a surfacetension of water, which is about 72 dyne/cm. Due to the properties suchas low viscosity and low surface tension, liquid carbon dioxide mayeasily penetrate into micro-pores existing between the nano-sized activematerial particles. Conversely, since most organic solvents or waterhave high surface tension and high viscosity, penetration intomicro-pores existing between the nano-sized active material particles isdifficult. In addition, liquid carbon dioxide is a liquid capable ofdissolving the carbon precursor organic material. Since liquid carbondioxide has a high density of about 0.7 g/cm³ to about 0.8 g/cm³ at roomtemperature, it has high solubility with respect to various kinds ofcarbon precursor organic materials.

Accordingly, when the carbon precursors are dissolved in liquid carbondioxide to coat the surfaces of the nano-sized active materialparticles, as compared to when the carbon precursors are dissolved in anorganic solvent or water to coat the surfaces of the nano-sized activematerial particles, the surfaces of the nano-sized active materialparticles may be uniformly coated with the carbon precursors. Inaddition, liquid carbon dioxide may be, unlike water or the organicsolvent, recovered and drained by reducing pressure after the coatingprocess. Accordingly, since a waste liquid treatment process after thecoating process can be omitted, the method of coating a secondarybattery active material with carbon using liquid carbon dioxideaccording to the example may be an economical and environmental friendlymethod.

In the coating solution, a concentration of the carbon precursors may bein the range of 1 to 80 wt %. When the concentration of the carbonprecursors is less than 1 wt %, the amount of carbon precursors is toosmall to uniformly coat the surfaces of the active material particles ofthe secondary battery. When the concentration of the carbon precursorsis more than 80 wt %, the viscosity of the carbon precursors is too highto penetrate into micro-pores of the secondary battery active material.In some embodiments, the concentration of the carbon precursors in thecoating solution may be in the range of 5 wt % to 40 wt %. In anotherembodiment, the concentration of the carbon precursors in the coatingsolution may be in the range of 25 wt % to 35 wt %.

The coating solution may be fabricated at a temperature of 0° C. to 30°C. under a pressure of 30 to 200 bar. In some embodiments, the coatingsolution may be, for example, produced at a temperature of 5° C. to 25°C. under a pressure of 50 to 100 bar. When the coating solution isfabricated at a temperature under 0° C., an additional cooling processmay be required to maintain a low temperature, which could beuneconomical. When the coating solution is fabricated at a pressureunder 30 bar, there is a problem in that solubility of the carbonprecursors with respect to liquid carbon dioxide decreases and carboncoating is not uniform. In addition, when the coating solution isfabricated at a temperature higher than 30° C. under a pressure higherthan 100 bars, additional costs for maintaining the high temperature andthe high pressure may be generated, and process reliability may bedecreased.

In the coating process (S20), the surface of secondary battery activematerial may be coated with the carbon precursors. In some embodiments,the surface of the secondary battery active material may be coated withthe carbon precursors by introducing the secondary battery activematerial in a coating vessel, adjusting a pressure in the coating vesselto be the same as the pressure of the solution vessel, and introducingthe coating solution into the coating vessel. Since the secondarybattery active material particles are coated with the precursor usingthe coating solution in which carbon is dissolved in liquid carbondioxide as described above, the coating solution may be penetrated intomicro-pores disposed between the secondary battery active materialparticles and, as a result, the surface of the secondary battery activematerial is uniformly coated with the carbon precursors.

As the secondary battery active material, any material that can be usedas an active material in the secondary battery may be used withoutlimitation. In some embodiments, one selected from titanium dioxide(TiO₂) or compounds represented by the following Formula 1 to Formula 3,may be used as the secondary battery active material.

LiMPO₄  [Formula 1]

Here, M represents one of manganese (Mn), iron (Fe), cobalt (Co), andnickel (Ni).

Li₂NSiO₄  [Formula 2]

Here, N represents iron (Fe) or manganese (Mn).

Li₄Ti₅O₁₂  [Formula 3]

The coating process (S20) may be performed under the same pressure asthe process of fabricating the coating solution (S10). In someembodiments, the coating solution may be introduced into the coatingvessel after adjusting the pressure in the coating vessel to be the sameas the pressure of the solution vessel by gradually introducing a carbondioxide gas into the coating vessel accommodating the secondary batteryactive material.

The coating process (S20) may be performed for 10 minutes to 48 hours,or 30 minutes to 24 hours, for example. When the coating process (S20)is performed for less than 10 minutes, the secondary battery activematerial may not be uniformed coated with the carbon precursors since acontact time between the secondary battery active material and thecarbon precursors is short. When the coating process (S20) is performedfor more than 48 hours, the productivity may be deteriorated since along period of contact between the secondary battery active material andthe carbon precursors needs to be maintained.

After the coating process (S20), a process of separating the secondarybattery active material coated with the carbon precursors from thecarbon precursors that are not coated on the surface of the secondarybattery active material but remaining in the liquid carbon dioxide phasemay be further included. The process may be performed by transferringthe liquid carbon dioxide solution to another high-pressure vessel byusing a high-pressure pump, and lowering the vessel pressure.

The process of removing the liquid carbon dioxide (S30) may be a processof removing the liquid carbon dioxide, which is a coating solvent, andthereby obtaining the secondary battery active material coated with thecarbon precursors.

In order to remove the liquid carbon dioxide, the liquid carbon dioxidemay be vaporized by gradually lowering a pressure of the vessel in whichthe liquid carbon dioxide and the secondary battery active material aremixed to atmospheric pressure. According to the present embodiment,since liquid carbon dioxide is removed by vaporization thereof, thesolvent may be environmentally friendly and easily removed.

In the sintering process (S40), the carbon precursors may be transformedto carbon. More specifically, the carbon precursors coating the surfaceof the secondary battery active material may be transformed to carbon bya heat treatment.

In some embodiments, the sintering process (S40) may be performed at atemperature of 200° C. to 800° C., or in the range of 500° C. to 600° C.When the sintering temperature is lower than 200° C., the carbonprecursors may not be fully sintered. When the sintering temperature ishigher than 800° C., particle sizes may increase due to agglomeration ofthe secondary battery active material and costs for sintering mayincrease due to maintenance of the high temperature.

Meanwhile, the sintering process (S40) may be performed for 30 minutesto 5 hours, or for 1 hour to 3 hours. When the sintering time is lessthan 30 minutes, there is a problem in that the carbon precursors maynot be fully sintered. When the sintering time is longer than 5 hours,particle sizes may increase due to agglomeration of the secondarybattery active material and costs for sintering may increase due tomaintenance of the high temperature.

In some embodiments, the sintering process (S40) may be performed in agas atmosphere of at least one of hydrogen, helium, neon, argon,krypton, xenon, and radon. For example, the sintering process (S40) maybe performed in a hydrogen atmosphere.

In addition, the sintering process (S40) may be performed in anatmosphere in which a hydrocarbon gas is present in addition to theabove-described gas. In this manner, when the sintering process (S40) isperformed in the atmosphere including the hydrocarbon gas, hydrocarbonmay be decomposed by the heat treatment to supply carbon to the surfaceof the secondary battery active material. Accordingly, a carbon coatinglayer may be formed in a wider area on the surface of the secondarybattery active material.

Meanwhile, in this example, crystallized active material particles maybe used as the secondary battery active material. In another example,active material particles, at least one portion of which is amorphous,may be used as the secondary battery active material. In this case, theamorphous portion may be crystallized by controlling the sinteringtemperature and time.

According to the method of carbon-coating the secondary battery activematerial using the liquid carbon dioxide, a uniform carbon coating layermay be formed on the surface of the secondary battery active material.In some embodiments, a weight ratio of carbon forming the carbon coatinglayer with respect to the total weight of the secondary battery activematerial coated with carbon may be about 1.6 to 5 wt %. When the carbonweight ratio is less than 1.6 wt % or more than 5 wt %, charge/dischargecapacity may not only be small but also be rapidly decreased ascharging-discharging cycles proceed.

The secondary battery active material coated with carbon fabricated inthis manner may be used to manufacture an electrode for a secondarybattery and the secondary battery including the electrode.

Hereinafter, various examples are shown and described; however, thepresent description should not be construed as limited to the examplesset forth herein.

Particle Characteristic Analysis of Secondary Battery Active MaterialCoated with Carbon Embodiment 1

As a secondary battery active material, 2 grams of LiFePO₄ particlesprepared by a solid-phase method were contained in a mesh and introducedin a high-pressure coating vessel. Then, a housing system including thehigh-pressure coating vessel was adjusted to be at a temperature of 15°C. by using a temperature controller, and a gaseous carbon dioxide wasintroduced into the high-pressure coating vessel to control a pressurein the high-pressure coating vessel to be 52 bar. As carbon precursors,sucrose octaacetate was introduced into a high-pressure solution vessel.Then, liquid carbon dioxide was introduced to the solution vessel toprepare a coating solution having a concentration of about 33 wt %.

The coating solution was transferred from the high-pressure solutionvessel to the high-pressure coating vessel using a high-pressure pump.In order for the coating solution to be sufficiently adsorbed to theLiFePO₄ particles, the LiFePO₄ particles were immersed in the coatingsolution for 30 minutes.

Next, the liquid carbon dioxide solution in the coating vessel wastransferred back into the high pressure solution vessel. Here, theLiFePO₄ particles are coated with the sucrose octaacetate by freemeniscus generated during the solution drainage. Gaseous carbon dioxideremaining in the high-pressure coating vessel was gradually evacuated ata rate of 15 ml/min by opening a flow-control valve, and the LiFePO₄particles coated with sucrose octaacetate were collected.

Next, the LiFePO₄ particles coated with sucrose octaacetate were heatedto 600° C. at a rate of 5° C./min while flowing a high purity (99.999%)argon gas containing 5% hydrogen, and maintained for 3 hours. Thereby,carbon layers were formed on surfaces of the LiFePO₄ particles.

Embodiment 2

Li₄Ti₅O₁₂ coated with a carbon layer was fabricated using the sameprocess as in Embodiment 1, except that Li₄Ti₅O₁₂ was used as thesecondary battery active material.

Embodiment 3

TiO₂ coated with a carbon layer was fabricated using the same process asin Embodiment 1, except that TiO₂ was used as the secondary batteryactive material.

Embodiment 4

Li₂FeSiO₄ coated with a carbon layer was fabricated using the sameprocess as in Embodiment 1, except that Li₂FeSiO₄ was used as thesecondary battery active material.

Comparative Example 1

Sucrose having a concentration of 12 wt % was dissolved in thricedistilled water, and LiFePO₄ particles were introduced in the sucrosesolution and thoroughly mixed. Slurry formed in this manner was dried ina vacuum oven at a temperature of 80° C. until moisture was removed. Thedried particles were finely ground using a mortar and pestle, and sievedthrough a sieve with 20 μm diameter. The sieved particles were heated to600° C. at a rate of 5° C./min while flowing a high purity (99.99%)argon gas containing 5% hydrogen, and maintained for 3 hours, to form acarbon layer.

Comparative Example 2

Li₄Ti₅O₁₂ coated with a carbon layer was fabricated using the sameprocess as in Comparative Example 1, except that Li₄Ti₅O₁₂ was used asthe secondary battery active material.

Comparative Example 3

TiO₂ coated with a carbon layer was fabricated using the same process asin Comparative Example 1, except that TiO₂ was used as the secondarybattery active material.

Comparative Example 4

Li₂FeSiO₄ coated with a carbon layer was fabricated using the sameprocess as in Comparative Example 1, except that Li₂FeSiO₄ was used asthe secondary battery active material.

Experimental Example

In order to analyze a phase state of the secondary battery activematerial particles fabricated by the method according to an embodimentof the present description, a Hitachi scanning electron microscopy (SEM)was used. A Rigaku X-ray diffractometer (XRD) was used to analyze thecrystalline structure of the particles, a Tecnai high-resolutiontransmission electron microscope (HR-TEM) was used to analyze the carbonlayer in the particles, and a Leco elemental analyzer (EA) was used toquantitatively analyze an actual carbon layer.

FIG. 2 is an XRD pattern graph of LiFePO₄ particles before coating withcarbon precursors in Embodiment 1, and FIG. 3 is an SEM photograph ofLiFePO₄ particles before coating with carbon precursors in Embodiment 1.

Referring to FIGS. 2 and 3, the LiFePO₄ particles before coating withthe carbon precursors have an olivine structure with high crystallinityand no impurities, particle sizes thereof are in the range of 100 nm to300 nm, and existence of pores between particles is confirmed.

FIG. 4 is an XRD pattern graph of LiFePO₄ particles coated with carbonprecursors in Embodiment 1, and FIG. 5 is an SEM photograph of LiFePO₄particles coated with carbon precursors in Embodiment 1.

Referring to FIGS. 4 and 5, the LiFePO₄ particles, even after coatingwith the carbon precursors, still have the olivine structure with highcrystallinity and no impurities. In addition, the carbon precursorscoating the LiFePO₄ particles have a nanotube shape having a diameter of20 nm to 50 nm and a length of 1 μm to 5 μm, and the carbon precursorsare tangled and spread between the LiFePO₄ particles.

FIG. 6 is an XRD pattern graph of LiFePO₄ particles coated with carbonlayers after sintering in Embodiment 1, and FIG. 7 is an SEM photographof LiFePO₄ particles coated with carbon layers after sintering inEmbodiment 1

Referring to FIGS. 6 and 7, the LiFePO₄ particles have an olivinestructure with high crystallinity and no impurities even after thesintering process. In addition, the nanotube-shaped carbon precursorshave been changed into the carbon layers by sintering. In particular,the LiFePO₄ particles coated with carbon layers have a similar shape tothe LiFePO₄ particles before coating with the carbon precursors in FIG.2. This implies that the carbon layers are uniformly formed.

FIGS. 8A and 8B are HR-TEM photographs of LiFePO₄ particles coated withcarbon layers after sintering in Embodiment 1, and it can be seen thatsurfaces of the LiFePO₄ particles have been uniformly coated with thecarbon layers to a thickness of 3 nm to 5 nm. Meanwhile, the amount ofcarbon in the LiFePO₄ particles coated with carbon layers was analyzedby EA, and it was 1.9 wt %.

FIG. 9 is an XRD pattern graph of LiFePO₄ particles before coating withcarbon precursors in Comparative Example 1, and FIG. 10 is an SEMphotograph of LiFePO₄ particles coated with carbon layers aftersintering fabricated in Comparative Example 1.

Referring to FIGS. 9 and 10, the LiFePO₄ particles before coating withthe carbon precursors have an olivine structure with the samecrystallinity as the LiFePO₄ particles before coating with the carbonprecursors according to Embodiment 1, and have a size and shape the sameas or similar to the size and shape of the LiFePO₄ particles beforecoating with the carbon precursors according to Embodiment 1.

FIGS. 11A and 11B are HR-TEM photographs of LiFePO₄ particles coatedwith the carbon layers after sintering in Comparative Example 1.

A mass of carbon having a diameter of about 1 μm, which does not coatthe LiFePO₄ particles, is observed and it is found that even the mass ofcarbon near the LiFePO₄ particles does not uniformly coat the LiFePO₄particles. The amount of carbon in the LiFePO₄ particles coated withcarbon layers was analyzed by EA, and it was 6.0 wt %.

Measurement of Discharge Capacity of Battery Embodiment 5

In order to analyze electrochemical properties of secondary batteryactive material particles coated with carbon, a secondary battery wasfabricated.

First, a cathode electrode was fabricated using the LiFePO₄ particlescoated with carbon according to Embodiment 1 as a positive electrodeactive material, acetylene black as a conductive agent, andpolyvinylidene fluoride as a binder. More specifically, in order tofabricate the cathode electrode, the active material, the conductiveagent, and the binder were mixed at a weight ratio of 85:10:5 in ann-methyl pyrrolidone solvent to fabricate slurry. The slurry was appliedon an aluminum foil in the form of a thin electrode plate having athickness of 250 μm, and dried in an oven at a temperature of 80° C. for6 hours or more.

Next, using a substance in which ethylene carbonate (EC), ethyl methylcarbonate (EMC), and diethyl carbonate (DEC) were mixed at a volumeratio of 1:1:1 and lithium phosphate hexafluoride (LiPF₆) was dissolved,as an electrolyte, a Li metal as a counter electrode, and theabove-described cathode electrode, a coin-type half-cell was fabricated.

Charging/discharging characteristics and cyclability of the fabricatedsecondary battery were examined while changing a charging/dischargingrate from 0.1 C to 30 C at a voltage from 2.5 to 4.3 V.

Embodiment 6

A secondary battery was fabricated using the same method as inEmbodiment 5, except that the anode electrode was fabricated usingLi₄Ti₅O₁₂ coated with carbon according to Embodiment 2 instead ofEmbodiment 1 as a cathode electrode active material, and using a copperfoil instead of the aluminum foil. Charging/discharging characteristicsand cyclability of the fabricated secondary battery were examined by thesame method as Embodiment 5.

Embodiment 7

A secondary battery was fabricated using the same method as inEmbodiment 5, except that the anode electrode was fabricated using TiO₂coated with carbon according to Embodiment 3 instead of Embodiment 1 asa cathode electrode active material, and using a copper foil instead ofthe aluminum foil. Charging/discharging characteristics and cyclabilityof the fabricated secondary battery were examined by the same method asEmbodiment 5.

Embodiment 8

A secondary battery was fabricated using the same method as inEmbodiment 5, except that the cathode electrode was fabricated usingLi₂FeSiO₄ coated with carbon according to Embodiment 4 instead ofEmbodiment 1 as a cathode electrode active material.Charging/discharging characteristics and cyclability of the fabricatedsecondary battery were examined by the same method as Embodiment 5.

Comparative Example 5

A secondary battery was fabricated using the same method as inEmbodiment 5, except that the cathode electrode was fabricated usingLiFePO₄ particles coated with carbon according to Comparative Example 1instead of Embodiment 1 as a cathode electrode active material.Charging/discharging characteristics and cyclability of the fabricatedsecondary battery were examined by the same method as Embodiment 5.

Comparative Example 6

A secondary battery was fabricated using the same method as inEmbodiment 5, except that the anode electrode was fabricated usingLi₄Ti₅O₁₂ coated with carbon according to Comparative Example 2 insteadof Embodiment 1 as a cathode electrode active material, and using acopper foil instead of the aluminum foil. Charging/dischargingcharacteristics and cyclability of the fabricated secondary battery wereexamined by the same method as Embodiment 5.

Comparative Example 7

A secondary battery was fabricated using the same method as inEmbodiment 5, except that the anode electrode was fabricated using TiO₂coated with carbon according to Comparative Example 3 instead ofEmbodiment 1 as a cathode electrode active material, and using a copperfoil instead of the aluminum foil. Charging/discharging characteristicsand cyclability of the fabricated secondary battery were examined by thesame method as Embodiment 5.

Comparative Example 8

A secondary battery was fabricated using the same method as inEmbodiment 5, except that the cathode electrode was fabricated usingLi₂FeSiO₄ coated with carbon according to Comparative Example 4 insteadof Embodiment 1 as a cathode electrode active material.Charging/discharging characteristics and cyclability of the fabricatedsecondary battery were examined by the same method as Embodiment 5.

Experimental Example

Table 1 illustrates results of measuring discharge capacity of thesecondary batteries fabricated according to Embodiment 5 to Embodiment 8of the present description, and Comparative Example 5 to ComparativeExample 8 after 10 cycles and 100 cycles at a rate of 0.1 C.

TABLE 1 Discharging Discharging Actual Capacity/Actual Capacity/ActualAmount of Discharging Discharging Amount Amount Carbon Capacity afterCapacity after of Carbon of Carbon Measured by 10 cycles at 100 cyclesat after 10 after 100 EA 0.1 C 0.1 C cycles at 0.1 C cycles at 0.1 CDivision (wt %) (mAh/g) (mAh/g) (mAh/g/wt %) (mAh/g/wt %) Embodiment 51.9 173 170 91.1 89.5 Embodiment 6 1.8 174 173 96.7 96.1 Embodiment 72.0 220 218 110 109 Embodiment8 1.9 300 295 157.9 155.3 Comparative 6.0148 135 24.7 22.5 Example 5 Comparative 5.8 165 155 28.4 26.7 Example 6Comparative 6.2 180 162 29.0 26.1 Example 7 Comparative 6.1 255 221 41.836.2 Example 8

As listed in [Table 1], although the amount of carbon in the secondarybatteries fabricated according to Embodiment 5 to Embodiment 8 of thepresent description is significantly lower than the amount of carbon inthe secondary batteries fabricated according to Comparative Example 5 toComparative Example 8, the former has high discharge capacity andundergoes small changes in the discharge capacity in accordance with theincrease of charge/discharge cycles.

More specifically, when comparing the discharge capacity per unit massof the secondary batteries fabricated according to Embodiment 5 toEmbodiment 8 of the present description with that of the secondarybatteries fabricated according to Comparative Example 5 to ComparativeExample 8 by dividing the discharge capacity measured after 10 cycles ata rate of 0.1 C by the actual amount of carbon, it can be seen that thesecondary batteries, active materials of which are coated with carbonusing liquid carbon dioxide, fabricated according to Embodiment 5 toEmbodiment 8 of the present description have remarkably excellentdischarge capacity. In addition, when comparing the discharge capacityper unit mass of the secondary batteries fabricated according toEmbodiment 5 to Embodiment 8 of the present description with that of thesecondary batteries fabricated according to Comparative Example 5 toComparative Example 8 by dividing the discharge capacity measured after100 cycles at a rate of 0.1 C by the actual amount of carbon, it canalso be seen that the secondary batteries, active materials of which arecoated with carbon using liquid carbon dioxide, fabricated according toEmbodiment 5 to Embodiment 8 of the present description have remarkablyexcellent discharge capacity.

FIG. 12 is an initial charge/discharge curve graph of LiFePO₄ particlesfabricated by a solid-phase method before carbon-coating. The initialdischarge capacity at the rate of 0.1 C is 90 mAh/g, which issignificantly lower than 170 mAh/g, which is a theoretical dischargecapacity of the LiFePO₄ particles.

FIG. 13 is an initial charge/discharge curve graph of LiFePO₄ particlescoated with carbon according to Embodiment 5 of the present description.The initial discharge capacity at a low charge/discharge rate of 0.1 Cis 173.5 mAh/g, which is slightly higher than the theoretical dischargecapacity of the LiFePO₄ particles. In addition, as the charge/dischargerate increases in the order of 0.2 C, 0.5 C, 1 C, 2 C, 5 C, 10 C, 20 C,and 30 C, the discharge capacity decreases in the order of 158 mAh/g,150 mAh/g, 141 mAh/g, 133 mAh/g, 120 mAh/g, 100 mAh/g, 82 mAh/g, and 72mAh/g.

FIG. 14 an initial charge/discharge curve graph of LiFePO₄ particlescoated with carbon according to Comparative Example 5. The initialdischarge capacity is 152.0 mAh/g, which is significantly lower than thedischarge capacity of the carbon-coated LiFePO₄ particles according toEmbodiment 5 of the present description. In addition, as thecharge/discharge rate increases in the order of 0.2 C, 0.5 C, 1 C, 2 C,5 C, 10 C, 20 C, and 30 C, the discharge capacity decreases in the orderof 145 mAh/g, 141 mAh/g, 134 mAh/g, 125 mAh/g, 111 mAh/g, 62 mAh/g, 39mAh/g, and 22 mAh/g. That is, the discharge capacity of carbon-coatedLiFePO₄ particles prepared according to Comparative Example 5 issignificantly lower than that according to Embodiment 1 of the presentdescription even at a high charge/discharge rate.

In FIG. 15, (a) is a graph illustrating the charge/discharge capacityversus the number of cycles of LiFePO₄ particles coated with carbonaccording to Embodiment 5, and (b) is a graph illustrating thecharge/discharge capacity versus the number of cycles of LiFePO₄particles coated with carbon according to Comparative Example 5. Here,since LiFePO₄ particles which are not coated with the carbon layer haveno carbon for improving electrical conductivity, the discharge capacityrapidly decreases from 90 mAh/g to 76 mAh/g after 10 cycles at 0.1 C.

In addition, Embodiment 5 shows excellent charge/dischargecharacteristics, that is, the initial charge/discharge capacity at 0.1 Cis 174 mAh/g and the charge/discharge capacity after 10 cycles at 0.1 Cis 173 mAh/g. On the contrary, Comparative Example 5 shows significantlylow charge/discharge characteristics compared to Embodiment 5, that is,the initial charge/discharge capacity at 0.1 C is 151 mAh/g and thecharge/discharge capacity after 10 cycles at 0.1 C is 148 mAh/g.

In addition, the discharge capacity in Embodiment 5 is maintained at 73mAh/g at a high rate of 30 C and recovered to 167 mAh/g when the rate isreduced to 0.2 C, while the discharge capacity in Comparative Example 5is 17 mAh/g at a high rate of 30 C, which is significantly low comparedto the discharge capacity in Embodiment 5.

Meanwhile, in the secondary battery active material coated with carbon,the following experiments were performed in order to check the effect ofthe weight ratio of a carbon coating layer on the charge/dischargecapacity of a secondary battery.

Embodiment 9

Surfaces of LiFePO₄ particles were coated with sucrose octaacetate byimmersing the LiFePO₄ particles fabricated by a solid-phase method in acoating solution in which sucrose octaacetate was dissolved in liquidcarbon dioxide for 30 minutes. Next, the LiFePO₄ particles coated withsucrose octaacetate were sintered in a furnace at 600° C. for 3 hours toform carbon layers on the surfaces of the LiFePO₄ particles. In thiscase, a weight ratio of the carbon coating layer with respect to thetotal weight of the LiFePO₄ particles coated with carbon was 0.9 wt %,measured by EA.

A cathode electrode was manufactured by using the LiFePO₄ particlescoated with carbon, fabricated in the above-described manner, as acathode electrode active material, acetylene black as a conductiveagent, and polyvinylidene fluoride as a binder.

Next, using a substance in which EC, EMC, and DEC were mixed at a volumeratio of 1:1:1 and LiPF₆ was dissolved, as an electrolyte and a Li metalas a counter electrode, together with the above-described cathodeelectrode, a coin-type half-cell was fabricated.

Embodiment 10

A cathode electrode active material was fabricated using the same methodas in Embodiment 9, except that a coating solution in which sucroseoctaacetate was dissolved in liquid carbon dioxide at a concentration ofabout 20 wt % was used. Then, a secondary battery was manufactured usingthe cathode electrode active material by the same method as inEmbodiment 9. In this case, a weight ratio of the carbon coating layerwith respect to the total weight of the LiFePO₄ particles coated withcarbon was 1.4 wt %, measured by EA.

Embodiment 11

A cathode electrode active material was fabricated using the same methodas in Embodiment 9, except that a coating solution in which sucroseoctaacetate was dissolved in liquid carbon dioxide at a concentration ofabout 30 wt % was used. Then, a secondary battery was manufactured usingthe cathode electrode active material by the same method as inEmbodiment 9. In this case, a weight ratio of the carbon coating layerwith respect to the total weight of the LiFePO₄ particles coated withcarbon was 1.9 wt %, measured by EA.

Embodiment 12

A cathode electrode active material was fabricated using the same methodas in Embodiment 9, except that a coating solution in which sucroseoctaacetate was dissolved in liquid carbon dioxide at a concentration ofabout 40 wt % was used. Then, a secondary battery was manufactured usingthe cathode electrode active material by the same method as inEmbodiment 9. In this case, a weight ratio of the carbon coating layerwith respect to the total weight of the LiFePO₄ particles coated withcarbon was 6.0 wt %, measured by EA.

Comparative Example 9

A secondary battery was manufactured using the same method as inEmbodiment 9, except that a cathode electrode active material was formedby a solid-phase method and LiFePO₄ particles without being coated withcarbon.

Experimental Example

FIG. 16 is a graph for describing changes in charge/discharge capacityaccording to the weight ratio of a carbon coating layer.

Referring to FIG. 16, the secondary batteries manufactured according toEmbodiment 9 to Embodiment 12, as compared to Comparative Example 9,have a remarkably high charge/discharge capacity and undergosignificantly smaller changes in discharge capacity in accordance withthe increase of charge/discharge cycles.

In addition, in Embodiment 9 to Embodiment 12, when the weight ratio ofthe carbon layer is 1.9 wt % (Embodiment 11), the highestcharge/discharge capacity is shown. More specifically, thecharge/discharge capacity is relatively low (about 140 mAh/g) when theweight ratios of the carbon layer are 0.9 wt % and 1.4 wt %, while thecharge/discharge capacity is relatively high (about 170 mAh/g) when theweight ratio of the carbon layer is increased to 1.9 wt %. However, thecharge/discharge capacity may again be decreased (150 mAh/g) when weightratio of the carbon layer is increased to 6.0 wt %.

In summary, in order to obtain high charge/discharge capacity in thesecondary battery active material coated with carbon, the weight ratioof the carbon layer may be, for example, about 1.6 to 5 wt %.

FIG. 17 illustrates an apparatus for carbon-coating a secondary batteryactive material using a liquid carbon dioxide according to an embodimentof the present description.

Referring to FIG. 17, carbon precursors are introduced in ahigh-pressure solution vessel 40 and a temperature of the high-pressuresolution vessel 40 is appropriately controlled using temperaturecontrollers 20 and 21. Then, liquid carbon dioxide is introduced in thehigh-pressure solution vessel 40 and mixed with the carbon precursorsusing a magnetic bar so that the carbon precursors are sufficientlydissolved in the liquid carbon dioxide to form a precursor solution.

A secondary battery active material in a mesh 42 is introduced in ahigh-pressure coating vessel 41, and a pressure is applied to thesecondary battery active material by introducing gaseous carbon dioxidein the high-pressure coating vessel 41.

After a first flow control valve 52 is opened to form a flow of gaseouscarbon dioxide in the high-pressure coating vessel 41, the precursorsolution in the high-pressure solution vessel 40 is transferred into thehigh-pressure coating vessel 41 containing the secondary battery activematerial. A second flow control valve 53 is controlled to gradually flowthe liquid carbon dioxide solution back into the high-pressure container40 after a sufficient time is passed so that surfaces of secondarybattery active material are fully immersed in the precursor solution.Next, the gaseous carbon dioxide remaining in the high pressure coatingvessel 41 is slowly evacuated using a third flow control valve 51.

The secondary battery active material particles coated with the carbonprecursors are collected from the high pressure coating vessel 41, andsintered to form a carbon layer. Accordingly, a lithium secondarybattery active material having excellent electrical conductivity andcharge/discharge characteristics may be manufactured.

According to the embodiment of the present description, a carbon coatinglayer may be uniformly formed on a surface of a secondary battery activematerial by dissolving a carbon precursor in liquid carbon dioxidehaving excellent solubility in an organic material and low surfacetension and low viscosity, coating the surface of the secondary batteryactive material with the carbon precursor using the liquid carbondioxide, and performing a heat treatment to transform the carbonprecursor coating on the secondary battery active material to the carbonlayer. Since liquid carbon dioxide easily penetrates micro-pores betweennano-sized particles of the secondary battery active material, theentire surface of the secondary battery active material can be uniformlycoated with even using a small amount of carbon precursor.

While this disclosure includes specific examples, it will be apparent toone of ordinary skill in the art that various changes in form anddetails may be made in these examples without departing from the spiritand scope of the claims and their equivalents. The examples describedherein are to be considered in a descriptive sense only, and not forpurposes of limitation. Descriptions of features or aspects in eachexample are to be considered as being applicable to similar features oraspects in other examples. Suitable results may be achieved if thedescribed techniques are performed in a different order, and/or ifcomponents in a described system, architecture, device, or circuit arecombined in a different manner, and/or replaced or supplemented by othercomponents or their equivalents. Therefore, the scope of the disclosureis defined not by the detailed description, but by the claims and theirequivalents, and all variations within the scope of the claims and theirequivalents are to be construed as being included in the disclosure.

What is claimed is:
 1. A method of producing a carbon-coated secondarybattery active material, comprising: forming a coating solutioncomprising a carbon precursor dissolved in liquid carbon dioxide;coating a surface of a secondary battery active material with the carbonprecursor by immersing the secondary battery active material in thecoating solution; and sintering the secondary battery active materialcoated with the carbon precursor.
 2. The method of claim 1, furthercomprising removing the liquid carbon dioxide after coating thesecondary battery active material with the carbon precursor and beforesintering the secondary battery active material coated with the carbonprecursor.
 3. The method of claim 1, wherein the carbon precursorcomprises at least one selected from the group consisting of sucroseoctaacetate, fluorinated hydrocarbons, polyethylene glycol, acrylicacid, methacrylic acid, acrylamide, vinyl pyrrolidone, glycidylmethacrylate, and styrene.
 4. The method of claim 3, wherein the carbonprecursor comprises at least one functional group selected from thegroup consisting of a carboxyl group, a hydroxyl group, an epoxy group,an ester group, a thiol group, and a sulfonic group.
 5. The method ofclaim 3, wherein a concentration of the carbon precursor in the coatingsolution is in the range of 5 to 40 wt %.
 6. The method of claim 3,wherein the coating solution is formed at a temperature of 0 to 30° C.under a pressure of 30 to 200 bar.
 7. The method of claim 6, wherein thecoating of the secondary battery active material is performed under apressure of 30 to 200 bar.
 8. The method of claim 1, wherein thesintering of the secondary battery active material is performed at atemperature of 200 to 800° C. for 1 to 3 hours.
 9. The method of claim8, wherein the sintering of the secondary battery active material isperformed in an atmosphere comprising one or more gases selected fromthe group consisting of hydrogen, helium, neon, argon, krypton, xenon,and radon.
 10. The method of claim 8, wherein the sintering of thesecondary battery active material is performed in an atmospherecomprising a hydrocarbon gas.
 11. The method of claim 8, wherein thesecondary battery active material comprises at least one selected fromthe group consisting of titanium dioxide, a compound represented byFormula 1, a compound represented by Formula 2, and a compoundrepresented by Formula 3, wherein:LiMPO₄  [Formula 1] (wherein M is one of manganese (Mn), iron (Fe),cobalt (Co), and nickel (Ni))Li₂NSiO₄  [Formula 2] (wherein N is iron (Fe) or manganese (Mn))Li₄Ti₅O₁₂.  [Formula 3]
 12. The method of claim 11, wherein thesecondary battery active material before sintering comprises at leastone amorphous portion, and the amorphous portion is changed to acrystalline portion during the sintering of the secondary battery activematerial.
 13. A secondary battery active material, comprising a carboncoating layer coating a surface thereof, wherein a weight ratio of thecarbon coating layer with respect to a total weight is 1.6 to 5 wt %.14. A method of producing a carbon-coated battery active material,comprising: mixing a carbon precursor with liquid carbon dioxide toproduce a coating solution comprising a carbon material; coating abattery active material with the carbon material by applying the coatingsolution to the battery active material; and sintering the coatedbattery active material to obtain the carbon-coated battery activematerial.
 15. The method of claim 14, wherein the carbon precursorcomprises at least one selected from the group consisting of sucroseoctaacetate, fluorinated hydrocarbons, polyethylene glycol, acrylicacid, methacrylic acid, acrylamide, vinyl pyrrolidone, glycidylmethacrylate, and styrene.
 16. The method of claim 14, wherein thesecondary battery active material comprises particles with an averagediameter between 10 to 100 μm, the particles comprising at least oneselected from the group consisting of titanium dioxide or a lithiumcomposite metal oxide including manganese, iron, cobalt, nickel ortitanium.